The present invention relates in general to a transmission system for transmitting drive power from a drive energy source to a driven load. The invention relates particularly to a transmission system for use in a motor vehicle, where the drive energy source is the engine of the vehicle and where the driven load can be considered as being the vehicle itself or its wheels. In the following, the invention will be explained for the particular application of a transmission system in a motor vehicle, but it is explicitly noted that the invention is not restricted to such application. For instance, it is possible that the driven load is a combustion engine, an electrical motor, a generator, a gas turbine, etc.
In motorised vehicles, it is common practice to transfer the energy from the drive energy source such as a combustion engine to the wheels through the intermediary of a transmission system. One of the purposes of the transmission system is to allow the output shaft of the engine and the wheels to rotate with mutually different rotational speeds. The ratio of the rotational speed of an input member of the transmission system to the rotational speed of an output member of the transmission system, indicated as transmission factor, can be changed manually or automatically, depending on the type of transmission system, and in particular the transmission factor may be continuously variable.
In automotive engineering, it is an ongoing objective to reduce energy consumption when the vehicle is in use. One way of achieving this objective is to minimise the size of the drive energy source. This reduces the amount of weight to be moved and provides an optimal use of the driver energy source and, if the drive energy source is an internal combustion engine, it usually provides better energy conversion efficiency. However, a disadvantage of smaller drive energy sources, especially smaller combustion engines, is that such drive energy source is only capable of providing adequate drive power at relatively high revolutions per minute. As a consequence, relatively small drive power sources have a bad load response, i.e. an unfavourable acceleration delay, because it is necessary to speed up the engine from a momentary engine speed to a higher engine speed in order to be able to deliver the required amount of drive power.
Therefore, it is an object of the present invention to provide a mechanical transmission system which is capable of assisting the engine in speeding up when acceleration is required.
In practice, a transmission system which is intended to assist the engine in speeding up is already commercially available from Honda; it is the so-called Integrated Motor Assist System. This known transmission system comprises a specific electrical motor coupled in series with a continuously variable transmission between a drive unit comprised of a combustion engine and a driven load comprised of the wheels of the vehicle, while further this known transmission system comprises a specific electrical accumulator. When acceleration is required, the relatively small combustion engine would give insufficient acceleration response by itself, but is assisted by the electrical motor which receives its energy from the electrical accumulator or battery. This battery is fed by the engine during less requiring operating conditions (and/or by absorbing breaking energy). Thus, when acceleration is required, the wheels are driven from two sources: the one source being the combustion engine, the other source being the electrical motor powered by the electrical accumulator. Thanks to the presence of the electrical motor and the electrical accumulator, the internal combustion engine can be chosen to be one with relatively low maximum engine torque, i.e. usually a relatively small engine. However, disadvantages of this known construction relate to the use of electrical energy, the necessity of converting kinetic energy to electrical energy vice versa, and the weight and costs of the electrical installation.
Therefore, it is a further object of the present invention to solve said disadvantages. More particularly, it is an object of the invention to provide a transmission system enabling the use of energy saving and relatively light drive energy sources (engines) while maintaining the advantage of using conventional, i.e. mechanical, technology, while further keeping the costs and weight of the transmission system relatively low.
According to an important feature of the present invention, the transmission system comprises an epicyclic gearing with three rotational members, wherein a first rotational member is coupled to an input of the transmission system, wherein a second rotational member is coupled to an output of the transmission system, and wherein a third rotational member is provided with a flywheel function, i.e. it is constructed as a flywheel or operatively connected with a flywheel. The epicyclic gearing may be of any type known per se, but preferably the epicyclic gearing comprises a planetary stage. Further the transmission system comprises a main transmission coupled between said input and said output of the transmission system, resulting in a desired ratio between the first and second rotational members of the epicyclic gearing being defined. In such transmission system according to the invention, under normal drive conditions the flywheel will absorb kinetic energy, while further the kinetic energy of the flywheel will be released towards either or both of the input and output of the transmission system during acceleration, thereby assisting the drive energy source. The construction is favourable in that it uses conventional technology with readily available components in a relative simple configuration. Further, the advantages of the invention can be achieved with a relatively low weight of the flywheel to be used. Further, the construction of the invention can be used in combination with any of the known types of transmission.
It is noted that it is known per se to use a flywheel in a drive line. In configurations where such flywheel is mounted directly of an input shaft of the transmission system, such configuration cannot be used to assist an engine in raising its rotational speed, since the flywheel is only capable of delivering energy while slowing down. In configurations where such flywheel is used to store kinetic energy when the vehicle is slowing down and to supply driving force to the vehicle when the vehicle is accelerating, it is necessary to equip the flywheel with a controlled coupling/uncoupling device, which is provided with actuation elements, which require actuation energy, etc, such that the construction is rather complicated. Further, when a flywheel is intended for supplying driving force to a vehicle, the flywheel must be relatively large and heavy, and must be operated at relatively high rotational speeds, however, the flywheel is used as a buffer of kinetic energy for the vehicle, i.e. when the vehicle is slowing down, kinetic energy of the vehicle is used to speed up the flywheel, while it is intended to accelerate the vehicle, rotational energy from the flywheel is transferred to the vehicle, causing the flywheel to slow down.
Further, it is noted that the use of epicyclic gearing in general, and the use of planetary stages in particular, in transmission systems is known per se. However, the purpose of such use differs from the purpose of the present invention in that normally the epicyclic gearing is applied as a fixed transmission ratio for improvement of the efficiency of a continuously variable transmission used in combination with the gearing or to change the ratio coverage of a transmission used in combination with the gearing. For instance, in the construction disclosed in WO96/35063, a planetary stage is operatively provided between an engine and a load, and a continuously variable transmission is arranged between the pinion carrier of the planetary stage and either the sun gear or the ring gear.
Further, it is also known per se in the art to use epicyclic gearings for increasing the overall transmission efficiency while reducing the ratio coverage or, vice versa, to increase ratio coverage to the detriment of the overall transmission efficiency.